/Engine/Plugins/Editor/AssetManagerEditor/Source/AssetManagerEditor/Public/ReferenceViewer/EdGraph_ReferenceViewer.h
If you're extending the reference viewer via GetAllGraphEditorContextMenuExtender you can use this to get the list of selected assets to use in your menu extender
bool GetSelectedAssetsForMenuExtender ( const class [UEdGraphNode](API\Runtime\Engine\EdGraph\UEdGraphNode) * Node, [TArray](API\Runtime\Core\Containers\TArray)< [FAssetIdentifier](API\Runtime\CoreUObject\AssetRegistry\FAssetIdentifier) > & SelectedAssets ) const